Topic 3: Raising Exceptions

1. Introduction

In Python, not only can exceptions be handled, but they can also be triggered manually using the raise statement. Raising exceptions is essential when you want to signal the occurrence of an anomalous event that should be addressed, either by the immediate code or by the calling code.

2. Why Raise Exceptions?

Raising exceptions can serve multiple purposes:

  • Validation: Enforcing certain conditions or invariants in the code.
  • Signaling: Notifying that a special, often rare, condition has occurred.
  • Debugging: Explicitly catching problematic states that are hard to detect.

3. Using raise

The basic syntax of raise is straightforward. You can raise a built-in exception or a custom one.


if some_condition: raise Exception("This is an error message.")

4. Raising Specific Exceptions

Python has several built-in exceptions, such as ValueError, TypeError, and RuntimeError. You can raise these with custom messages that provide insight into the problem.


def set_age(age): if age < 0: raise ValueError("Age cannot be negative!") # ... rest of the code

5. Reraising Exceptions

You might sometimes want to catch exceptions, perform some actions (like logging), and then reraise the same exception. This can be done using raise without an argument in an except block.


try: # Some operation that can raise an exception x = 1/0 except ZeroDivisionError: print("Logging the error...") raise # reraises the caught exception

6. Custom Exceptions

For more specific error handling, you can create your own exception classes. Custom exceptions should typically derive from the Exception class or a derived class.


class NegativeAgeError(Exception): """Raised when age is negative.""" def set_age(age): if age < 0: raise NegativeAgeError("Age cannot be negative!")

7. raise ... from Syntax

In Python 3, if you’re raising a new exception in response to another exception (for instance, translating exceptions), you might want to retain the original exception’s context. The raise ... from syntax allows this.


try: x = int("not a number") except ValueError as e: raise TypeError("A number was expected!") from e

In this case, the TypeError will indicate that it was directly caused by the ValueError.

8. Conclusion

Raising exceptions is a powerful tool in the Python programmer’s toolkit. It allows you to enforce constraints, signal errors, and ensure that problematic states are caught and appropriately addressed. By understanding how and when to raise exceptions, you can make your programs more robust and maintainable.